Vehicular Traffic Surveillance Doppler Radar System

ABSTRACT

A vehicular traffic surveillance Doppler radar system and method for use of the same are disclosed. In one embodiment, the system comprises a modulation circuit portion for generating modulated FM signals. An antenna circuit portion transmits the modulated FM signals to a target and receives the reflected modulated FM signals therefrom. A ranging circuit portion performs a quadrature demodulation on the reflected modulated FM signals and determines a range measurement based upon phase angle measurements derived therefrom.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 11/279,383 entitled “Vehicular Traffic Surveillance DopplerRadar System” and filed on Apr. 11, 2006, in the name of John L. Aker;which is a continuation of U.S. patent application Ser. No. 11/059,476,entitled “Vehicular Traffic Surveillance Doppler Radar System”, filed onFeb. 16, 2005, and issued on May 2, 2006 as U.S. Pat. No. 7,038,614 inthe name of John L. Aker; both of which are hereby incorporated byreference for all purposes.

TECHNICAL FIELD OF THE INVENTION

This invention relates, in general, to police radar systems and, inparticular, to a vehicular traffic surveillance Doppler radar systemthat is operable to determine the range of a target.

BACKGROUND OF THE INVENTION

The role of radar in traffic safety enforcement is widespread throughoutthe United States and the principal tool for police traffic surveillanceis Doppler radar. In a police Doppler radar system, an emitted microwavefrequency signal is reflected from a target vehicle, causing a change inthe frequency of the signal in proportion to a component of the velocityof the target vehicle. The Doppler radar system measures the frequencydifferential and scales the measurement to miles per hour, for example,in order to display the velocity of the target vehicle to a policeman orother Doppler radar system operator. Using the existing frequencydifferential scheme, conventional police Doppler radar systems arecapable of a high degree of accuracy with regard to vehicle speedmeasurements in environments having one target vehicle.

It has been found, however, that the existing police Doppler radarsystems are not necessarily successful in environments having multiplevehicles in position to reflect the radar signal. In particular,identification of the vehicle whose speed is being displayed whenmultiple vehicles are in a position to reflect the radar signal hasproven difficult due to “look-past error,” which occurs in situationswhere the intended target vehicle in the foreground has a significantlysmaller radar cross-section than an unintended target vehicle in thebackground. Accordingly, further improvements are warranted in the fieldof traffic surveillance Doppler radar systems.

SUMMARY OF THE INVENTION

A system and method are disclosed that provide for a vehicular trafficsurveillance Doppler radar which substantially eliminates look-pasterror by determining the range and speed of vehicles in multiple vehicleenvironments. In particular, the range of the target may be utilized ina comparative fashion in a multi-vehicle environment to determine whichvehicle is closer or the closest. Also, the range of the target may beutilized in conjunction with the speed and heading of the target todetermine the risk of a collision between the target and a patrolvehicle.

In one embodiment, a system comprises a modulation circuit portion forgenerating modulated FM signals, such as double-modulated FM signals. Anantenna circuit portion transmits the modulated FM signals to a targetand receives the reflected modulated FM signals therefrom. A rangingcircuit portion, which may include a quadrature circuit portion and aprocessing circuit portion, performs a quadrature demodulation on thereflected modulated FM signals and determines a range measurement basedupon phase angle measurements derived therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent invention, reference is now made to the detailed description ofthe invention along with the accompanying figures in which correspondingnumerals in the different figures refer to corresponding parts and inwhich:

FIG. 1A depicts a schematic illustration of a multiple vehicleenvironment wherein one embodiment of a system for traffic surveillanceis being utilized;

FIG. 1B depicts a schematic illustration of a single vehicle environmentwherein one embodiment of a system for determining a range of a targetis being utilized;

FIG. 2 depicts a schematic diagram of one embodiment of the vehiculartraffic surveillance Doppler radar system;

FIG. 3 depicts a schematic diagram of one embodiment of a quadraturemixer for the vehicular traffic surveillance Doppler radar system;

FIG. 4 depicts a flow chart of one embodiment of a method fordetermining a range of a target and avoiding a collision;

FIGS. 5A and 5B together depict a flow chart of another embodiment of amethod for determining a range of a target; and

FIGS. 6A, 6B, and 6C together depict a flow chart of a furtherembodiment of a method for determining a range of a target.

DETAILED DESCRIPTION OF THE INVENTION

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts whichcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention, and do not delimit the scope of the presentinvention.

Referring initially to FIG. 1A, therein is depicted an environment 10having multiple vehicles wherein one embodiment of the vehicular trafficsurveillance Doppler radar system is being utilized. A highway 12includes a northbound lane 14 having a speed limit of 55 mph as depictedby speed limit sign 16 and a southbound lane 18 having a speed limit of55 mph as depicted by speed limit sign 20. A vehicle is traveling in thenorthbound lane 14 at a speed of 50 mph as indicated by the northboundarrow and number “50” proximate to the front portion of the vehicle 22.Vehicles 24, 26, and are traveling in the southbound lane 18 at speedsof 75 mph, 55 mph, and 40 mph, respectively. A patrol vehicle 30equipped with a vehicular traffic surveillance Doppler radar system isstationary and facing north in a location that is proximate to thenorthbound lane 14. The vehicle 22 is approaching the patrol vehicle 30quickly. Additionally, with respect to the position of patrol vehicle30, vehicles 24 and 26 are positioned such that conditions are presentfor look-past error.

A front-facing antenna 32 and a rear-facing antenna are mounted on thepatrol vehicle 30 for surveying traffic. A control panel 36 isassociated with the patrol vehicle 30 and preferably secured to thedashboard in the interior of the patrol vehicle 30. The control panel 36includes a power button 38, a speaker 40, and a collision avoidanceindicator 42. Displays 44 and 46 indicate the speeds (in mph) of theclosest vehicle and the faster vehicle, respectively, associated withthe front-facing antenna 32. Similarly, displays 48 and 50 indicate thespeeds (in mph) of the closest vehicle and the faster vehicle,respectively, associated with the rear-facing antenna 34. In instanceswhere only one vehicle is associated with the rear-facing antenna 34,the display 50 is empty. A display 52 indicates the speed (in mph) ofthe patrol vehicle 30. It should be appreciated that the displays 44-52may either be dedicated to displaying the indicators discussedhereinabove or configurable to provide other types of indications. Forexample, a display may be configured to provide a history of the speedof a particular targeted vehicle.

As illustrated, the patrol vehicle 30 is monitoring the traffic in boththe northbound lane 14 and southbound lane 18. With regard to bothantennas 32 and 34, the vehicular traffic surveillance Doppler radarsystem is in a stationary, closest and faster, approaching only mode. Inthis mode, the police vehicle 30 is stationary and monitoring the speedsof both the closest and faster vehicles approaching the police vehicle30. It should be appreciated, however, that other modes of operation arewithin the teachings of the present invention. By way of example, thefollowing table provides a non-exhaustive matrix of the more commonoperator selectable modes of the multi-mode radar system disclosedherein.

TABLE I Common Operator Selectable Modes Patrol Vehicle Type ofSignal(s) Receding/Approaching Stationary Closest and Faster ApproachingStationary Closest and Faster Receding or Approaching Stationary Closestand Faster Receding Stationary Closest Approaching Stationary ClosestReceding or Approaching Stationary Closest Receding Moving Closest andFaster Approaching Moving Closest and Faster Receding or ApproachingMoving Closest and Faster Receding Moving Closest Approaching MovingClosest Receding or Approaching Moving Closest Receding

With respect to the forward-facing antenna 32, the officer operating thevehicular traffic surveillance Doppler radar system is intending totarget vehicle 26 which is a small sports car having a relatively smallradar cross-section (σ₁). The forward-facing antenna 32 emitsdouble-modulated FM signals that spread less than ten degrees from anaxis of the forward-facing antenna 32. It should be appreciated thatalthough the present invention is described as utilizingdouble-modulated FM signals, other types of modulated FM signals may beutilized. For example, triple and higher order modulated FM signals maybe utilized. In one embodiment, the double-modulated FM signals are acontinuous wave signal transmission that is alternated between a firstand second frequency, which may be expressed as f₁ and f₂, respectively.The double-modulated FM signals reflect off of the vehicle 26 and thevehicle 24 which is a large truck having a relatively large radarcross-section (σ₂), wherein σ₂>>σ₁. Hence, the vehicle 24 has a strongerreflected signal than the vehicle 26 even though the vehicle 24 isfarther away from the patrol vehicle 30 than the vehicle 26. Thereflected double-modulated FM signals generated by the vehicles 24 and26 are received by the forward-facing antenna 32 and processed toresolve the multiple targets by determining the direction, speed, andrange of the targeted vehicles 24 and 26.

As will be discussed in further detail hereinbelow, the vehiculartraffic surveillance Doppler radar system receives the reflecteddouble-modulated FM signals and performs a quadrature demodulation onthe reflected double-modulated FM signals. In one embodiment, homodynereception is utilized wherein a voltage having the original carrierfrequency is generated and combined with the incoming reflecteddouble-modulated FM signals. The quadrature demodulated, reflected FMsignals are then converted to digital signals and a fast Fouriertransform (FFT) is performed that results in an approaching or closingtarget spectrum and a receding or opening target spectrum. In oneimplementation, a complex FFT is performed on the data. Analysis of theresulting spectra using the multi-direction sensing capabilities of theinstant police radar indicates that both of the vehicles 24 and 26 areapproaching. The frequency signal differentials associated with eachtarget are also analyzed to determine that the vehicle 24 is travelingat 75 mph and the vehicle 26 is traveling at 55 mph. The phase anglesignal differentials associated with each of the targets are analyzed todetermine that the vehicle 24 is at a range of R₂₄ and that the vehicle26 is at a range of R₂₆. As R₂₆<<R₂₄, the vehicle 26 is closer to thepatrol vehicle 30 than the vehicle 24. The speed of the closest vehicle,i.e., vehicle 26, is indicated at the display 44 and the speed of thefaster vehicle, i.e., vehicle 24, is indicated at display 46.

The police officer operating the vehicular traffic surveillance Dopplerradar system uses the displayed information to determine that theclosest target, which is vehicle 26, is traveling at 55 mph and a moredistant target, which is vehicle 24, is traveling at 75 mph. Existingradar systems assume that the strongest target is the closest target;namely, the vehicle 26 in the illustrated example. Accordingly, ifpatrol vehicle 30 had been equipped with an existing radar system, thenthe vehicle 26 could have appeared to have been traveling 75 mph in a 55mph zone. The vehicular traffic surveillance Doppler radar systempresented herein avoids this false positive due to look-past error bycalculating target ranges based upon the phase angle signaldifferentials associated with the targets rather than assuming signalstrength is indicative of range and, in particular, the strongest signalis from the closest vehicle.

With respect to the rear-facing antenna 34, the officer operating thevehicular traffic surveillance Doppler radar system is intending totarget vehicle 22. The emitted double-modulated FM signals reflect fromthe vehicle 22 and the vehicle 28 which is heading south in thesouthbound lane 18. The reflected double-modulated FM signals areprocessed to determine the direction, speed, and range of the targets.The vehicle 28 is receding from the patrol vehicle 30, so the speed andrange of the vehicle 28 are ignored since the vehicular trafficsurveillance Doppler radar system is in an approaching only mode. Thespeed, 50 mph, and range, 300 ft, of the vehicle 22 are determined andthe display 48 indicates that the closest vehicle is traveling at 50mph. The police officer uses the displayed information to determine thatvehicle 22 is traveling at 50 mph.

Further, the vehicular traffic surveillance Doppler radar systemincludes safety features that determine if conditions are safe for thepatrol vehicle 30 to pull-out in front of oncoming traffic based on thespeed and range of the oncoming vehicles. Based on the speed (50 mph)and the range (300 ft) of the vehicle 22, the vehicular trafficsurveillance Doppler radar system determines that conditions arehazardous and a collision with vehicle 22 is possible if the patrolvehicle 30 pulls into the northbound lane 14. In one implementation, toindicate that conditions are hazardous and a collision is possible, thevehicular traffic surveillance Doppler radar system provides a visualindication via collision avoidance indicator 42 and an audio indicationvia speaker 40 to the police officer operating the vehicular trafficsurveillance Doppler radar system. In one embodiment, the visualcollision avoidance indicator 42 and the audio indication via speaker 40are only enabled when the patrol vehicle 30 is in motion. In anotherembodiment, the operator of the patrol vehicle 30 may turn the collisionavoidance system OFF and ON as the system is needed.

FIG. 1B depicts the environment 10 of FIG. 1A wherein one embodiment ofa system for determining the range of a target vehicle is beingutilized. As illustrated, the patrol vehicle 30 is monitoring a vehicle54 traveling at a speed of 55 mph in the southbound lane 18.Forward-facing antenna 32 transmits a continuous FM signal to thevehicle 54 and receives a reflected double-modulated signal therefrom.The frequency signal differentials associated with the signals areanalyzed to determine that the vehicle is traveling at 55 mph. Inparticular, when the double-modulated FM signal is reflected from thevehicle 54, the frequency of the reflected double-modulated FM signal isshifted in proportion to a component of the velocity of the vehicle 54.The shift or frequency signal differential between the double-modulatedFM signal and the reflected double-modulated FM signal provides thespeed of vehicle 54. In instances where triple or higher order modulatedFM signals are utilized, mathematical “fold overs” are used to removephase ambiguities and distinguish between short and long range targetsso that target metrics such as speed, heading, and range can bedetermined.

Further, the phase angle signal differentials associated with thereflected double-modulated signal are analyzed to determine that thevehicle 54 is at a range of R₅₄ from the patrol vehicle 30. Morespecifically, the range, R₅₄, is related to the change or differentialin phase, Δ_(phase), in the reflected double-modulated signal by thefollowing range equation:

R ₅₄=(Δ_(phase) *C)/(4π(f ₂ −f ₁)), where

Δ_(phase)=φ₂−φ₁ for the phase angle signals (φ) in first and second setsof data (phase angle may be expressed in radians);

f₁ and f₂ are the frequencies utilized in the double-modulated signal(frequency may be expressed in gHz); and

c is the speed of light (186,282 miles/second).

Utilizing the relationships described by the range equation, the trafficsurveillance Doppler radar system determines the phase magnitude betweenthe phase angle signals (φ) to arrive at a target range. For example, ifthe change in phase, Δ_(phase), of the received reflecteddouble-modulated signal is +/−180° and f₁=34.7 gHz and f₂=34.7001 gHz,then the range, R₅₄, of the vehicle 54 is 2,460 ft (0.47 miles). Inmulti-vehicle environments, such as the environment described in FIG.1A, by calculating the range of each vehicle in the multi-vehicleenvironment, the Doppler radar system described herein is able todetermine which vehicle is the closest or which vehicle is closer.Additionally, the range of the target may be utilized in conjunctionwith the speed and the heading of the target to determine the risk of acollision between the target and a patrol vehicle.

More specifically, to determine the range, speed, and heading of thetarget vehicle, for example, the Doppler radar system sets themodulation state to frequency f₁ and a processor, such as a digitalsignal processor (DSP), within the Doppler radar system acquires a fullbuffer of data 56A for each of two quadrature demodulated channels. Oncethe buffer is full, a Hamming window 56B or other discrete Fouriertransform (DFT) window is applied to both channels of data. A complexFFT 56C is performed and two spectra 56D of Fourier components areoutputted, i.e., one for the approaching targets and one for thereceding targets. Targets that are heading towards the Doppler radar arelocated in the approaching spectra and targets that are heading awayfrom the Doppler radar are located in the receding spectra. The spectra56D are searched for the strongest target signals and data 56E, such asfrequency, signal strength, and phase angle value (φ₁), is stored foreach target signal. For example, with respect to the vehicle 54, theapproaching portion of spectra 56D is searched and frequency and phasevalue measurements are stored.

In one embodiment, at the end of the collection of the first buffer ofdata 56A and during the processing depicted by numerals 56B through 56E,the modulation state is set to frequency f₂ and a second buffer of data58A is collected before a sequence of data processing operations 58Bthrough 58E are executed which are analogous to the processingoperations 56B through 56E. In particular, the phase angle value (φ₂)for the second set of data is collected. Once the data 58E is collected,the phase angle values (φ₁ and φ₂) for the target signals in spectra 56Dand 58D that correspond to the vehicle 54 are compared and the change inphase angle, Δ_(phase), is utilized to calculate the range of thevehicle 54 as described by the range equation presented hereinabove.Additionally, to determine the speed of the vehicle 54, the frequency ofthe f₁ or f₂ signal is measured since the change in frequency isproportional to a component of the velocity of the target vehicle. Itshould be appreciated that the speed and the range of the vehicle may becalculated in any order or simultaneously.

FIG. 2 depicts one embodiment of the vehicular traffic surveillanceDoppler radar system which is generally designated 60. The vehiculartraffic surveillance Doppler radar system includes an antenna circuitportion 62, a quadrature circuit portion 64, a processing circuitportion 66, and a modulation circuit portion 68. The antenna circuitportion 62 includes an antenna 70 that transmits outgoing radar waves inthe form of double-modulated FM signals and receives reflecteddouble-modulated FM signals from stationary and moving objects includingintended and unintended target vehicles. A duplexer 72 guides theoutgoing double-modulated FM signals from the modulation circuit portion68 to antenna 70 and guides reflected radar waves received by antenna 70to the quadrature circuit portion 64.

The quadrature circuit portion 64 includes a quadrature mixer 74 coupledto the duplexer 72 in order to receive the reflected double-modulated FMsignals. The quadrature circuit portion 64 is also coupled to themodulation circuit portion 68 in order to receive a local oscillatorsignal. As will be explained in further detail in FIG. 3, the quadraturemixer 74 performs a quadrature demodulation by mixing the localoscillator signal with incoming RF of reflected radar waves in twoseparate mixers in two separate channels such that one channel isshifted by 90° relative to the other channel. The quadraturedemodulation results in a channel A signal that is driven to amplifier76 and a channel B signal that is driven to amplifier 78.

Preferably, amplifiers 76 and 78 are matched, low noise amplifiers. Theamplified channel A and B signals are driven to the processing circuitportion 66 and received by analog-to-digital (A/D) converters 80 and 82,respectively. The A/D converters 80 and 82 sample the analog signalsfrom amplifiers 76 and 78, respectively, and output the sampled signalsas digital data sampled signals on one or more transmission paths, suchas busses, infrared (IR) communication paths, or cables, connected to aDSP 84.

The DSP 84 processes the digital data samples from channels A and B byperforming a FFT thereon to develop the aforementioned approachingtarget Fourier spectrum and receding target Fourier spectrum. The targetFourier spectra are searched for targets and the direction each targetis traveling relative to the patrol vehicle is identified. Dataassociated with the spectra is further analyzed to determine the speedsof the identified targets based upon frequency signal differentialsassociated with the targets. Additional information regarding thedirection and speed sensing capabilities of the radar system of thepresent invention may be found in the following co-owed United Statespatents: (1) U.S. Pat. No. 6,198,427, entitled “Doppler Complex FFTPolice Radar With Direction Sensing Capability,” issued on Mar. 6, 2001in the names of Aker et al.; and (2) U.S. Pat. No. 6,646,591, entitled“Doppler Complex FFT Police Radar With Direction Sensing Capability,”issued on Nov. 22, 2003 in the names of Aker et al.; both of which arehereby incorporated by reference for all purposes.

As previously discussed, the range of the identified target iscalculated based upon a phase angle signal differential associated withthe target. In particular, the ranging circuit portion 98, which in oneembodiment includes the quadrature circuit portion 64 and the processingcircuit portion 66, determines the heading of the target as well as therange of the target. In one implementation, heading or direction isdetermined by comparing the phases or lead/lad relationship of twofrequencies. For example, a target at 1000 ft. might lead by 70 degreesif closing and lag by 70 degrees if opening. With respect to the rangeof the target, in regard to a particular target, the phase angle isarbitrary. However, the difference between the phase angle for theparticular target with respect to a first set of data and the phaseangle for the particular target with respect to a second set of data isindicative of the range between the police Doppler radar of the presentinvention and the particular target. Accordingly, the vehicularsurveillance Doppler radar system described herein permits targetdiscrimination between any two targets including two targets of the samespeed traveling in opposite directions.

Target metrics, such as direction or heading, speed, and range,determined by the DSP 84 are provided to the operator via an audiooutput 86 and a display 88, which, in one implementation, may be controlunit 36 of FIG. 1. An operator interface 90, which may include frontpanel or remote controls, provides for general operation of the systemincluding operator selectability of the aforementioned multiple modes ofoperation. It should be appreciated that the range measurements may bepresented in a comparative fashion where one vehicle is indicated to becloser or the closest relative to one or more other vehicles.

A D/A convertor 92 receives multiple digital signals from the DSP 84 andconverts these signals to a voltage which is supplied to a voltageconditioner 94 which may be a voltage regulator or varactor device, forexample. Operating in combination with the converter 92, the voltageconditioner 94 provides two voltages to an oscillator 96 that, in turn,generates the double-modulated FM signals. In particular, a frequencyversus voltage characteristic that is associated with the oscillator 96is utilized to generate two frequencies with only a relatively smalldifference in the applied voltages. In one embodiment, the DSP 84determines the required calibration and generates a calibration signalindicative of the voltages required to generate the desired frequencies.The calibration signal is received by the D/A converter 92, which incombination with the voltage conditioner 94, applies the two voltages tothe oscillator 96 that generate the desired double-modulated frequency.

Further information regarding the target range self-calibratingcapabilities of the radar system of the present invention may be foundin the following commonly owned, co-pending patent application: “Systemand Method for Calibrating a Vehicular Traffic Surveillance DopplerRadar,” filed on Feb. 16, 2005, assigned application Ser. No.11/059,474, and issued on Jun. 6, 2006 as U.S. Pat. No. 7,057,550 in thename of John L. Aker; which is hereby incorporated by reference for allpurposes.

In one implementation, the oscillator 96 comprises a dielectricresonator oscillator (DRO) or a Gunn diode oscillator that utilizes anegative resistance property of bulk gallium arsenide (GaAs) to convertan applied DC voltage into microwave power. Further informationregarding the modulation circuit portion 68 may be found in thefollowing commonly owned, co-pending patent application: “ModulationCircuit for a Vehicular Traffic Surveillance Doppler Radar System,”filed on Feb. 16, 2005, assigned application Ser. No. 11/059,199, andissued on May 23, 2006 as U.S. Pat. No. 7,049,999 in the name of John L.Aker; which is hereby incorporated by reference for all purposes.

It should be appreciated by those skilled in the art that although aparticular arrangement of circuitry has been illustrated with respect tothe radar system of the present invention, the radar system of thepresent invention may comprise any combination of hardware, software,and firmware. In particular, each of the circuit portions 62, 64, 66,and 68 of the present invention may comprise any combination ofhardware, software, and firmware.

FIG. 3 depicts one embodiment of the quadrature mixer 74. As previouslydiscussed, the function of the quadrature mixer 74 is to mix localoscillator energy with incoming RF of reflected radar waves in twoseparate mixers in two separate channels and shift one channel 90°relative to the other channel. Incoming RF from the duplexer 72 isprovided to a channel A mixer 102 and a channel B mixer 104. Localoscillator (LO) energy arrives from oscillator 96 and is provided as asecond input to mixer 102. The local oscillator energy is also coupledto a second input of mixer 104 via a phase shifter 106 which, in theillustrated embodiment, is a 90° phase shifter. The phase shifter 106shifts the local oscillator signal by 90°, or any integer multiple of90°, in either direction relative to the local oscillator signal. In analternative embodiment, instead of shifting the local oscillator signalby 90°, the incoming RF can be shifted by 90° at the input of one mixerrelative to the same incoming RF at the input of the other mixer.

The 90° phase shift can be achieved in any known manner associated withquadrature demodulation. In the preferred embodiment, the 90° phaseshift is achieved by having a microwave transmission line which isone-quarter wavelength (at the frequency of operation) longer in thepath from the local oscillator or RF input to one mixer than it is inthe path to the other mixer. By way of example, other techniques such asreactive circuits or delay lines may also be used.

The mixers 102 and 104 modulate the local oscillator signals with theDoppler shifted RF signals reflected from stationary and moving objectsand output sum and difference frequencies on a channel A output line anda channel B output line. Low pass filters 108 and 110 are coupled to thechannel A and B output lines, respectively, in order to remove the uppersideband (local oscillator plus Doppler shifted RF) signals from each ofthe spectrum on the channel A and B output lines so that only thedifference frequencies are outputted. Preferably, to reduce errors andnoise, the mixers 102 and 104 and low pass filters 108 and 110 arematched as closely as possible since amplitude variations betweenchannels A and B may cause noise in the system.

FIG. 4 depicts one embodiment of a method for avoiding collisionsbetween a source vehicle, such as a law enforcement cruiser ormotorcycle, having a Doppler radar associated therewith and a targetvehicle. At block 120, double-modulated FM signals are transmitted fromthe Doppler radar to the target vehicle. The double-modulated signalsmay be the aforementioned double-modulated f₁ and f₂ FM signals. Atblock 122, the Doppler radar receives reflected double-modulated FMsignals from the target vehicle. At block 124, the target vehicle'srange is determined based upon phase angle measurements associated withthe reflected double-modulated FM signals. As part of determining therange, a quadrature demodulation may be performed on the reflecteddouble-modulated FM signals. At block 126, the target vehicle's speed isdetermined based upon frequency signal differentials associated with thetransmitted and reflected double-modulated FM signals. It should beappreciated that the operations presented in blocks 124 and 126 may beperformed simultaneously or in another order such as reverse order. Atthis time, the heading or direction fo the target vehicle with respectto the source vehicle may also be determined. At block 128, the risk ofcollision between the source vehicle and the target vehicle isdetermined. The risk of collision is related to several variablesincluding the speed of the source vehicle and the speed, heading, andrange of the target vehicle. For example, if the source vehicle, i.e.,patrol vehicle, is stationary and the target vehicle is approaching at50 mph at a range of 300 feet then the target vehicle will be passingthe patrol vehicle in approximately 4 seconds. Hence, the patrol vehicledoes not have enough time to merge into the approaching or oncomingtraffic and the Doppler radar notifies the operator that a risk ofcollision is present. It should be appreciated that the time thresholdfor safe merger/risk of collision is adjustable. For example, the timethreshold may be set for 10 seconds. Further, the time threshold may beadjusted in accordance with the time of day, road conditions, andweather, for example.

FIGS. 5A and 5B together depict another embodiment of a method fordetermining a range of a target. At block 140, the Doppler radar isoperating in a stationary patrol vehicle, closest target, receding orapproaching only mode that is implemented utilizing a relative distancemeasurement process. At block 142, the modulation state is set tofrequency f₁. At block 144, a first buffer of data is acquired that hastwo halves; namely, one for each of the quadrature shifted channels. Atwaiting block 146, once the end of the first buffer of data is reached,the methodology advances to block 148 wherein the modulation state isset to frequency f₂.

At block 150, the second buffer of data is collected. While this secondbuffer of data is being collected, the methodology continues to block152 where a Hamming window or other DFT window is applied to each halfof the first buffer of data in order to minimize end effects. At block154, a complex FFT is performed utilizing one-half of the buffer as thereal magnitude and utilizing the other half buffer as the imaginarymagnitude. This generates two spectra of Fourier components; namely, onespectra for approaching targets and one spectra for receding targets.These two spectra are searched at block 156 in order to find and store apredetermined number of strong signal target data values and relevantparameters such as frequency, signal strength, and phase value for eachtarget signal.

At waiting block 158, the methodology waits until the second buffer ofdata is acquired before advancing to block 160 where the modulationstate is returned to frequency f₁ and the next first buffer of data isacquired at block 162. At blocks 164 through 168, while the data of thefirst buffer is being collected, the data of the second buffer isanalyzed. Briefly, similar to the operations of blocks 152 through 156,at block 164, the end effects of the data are minimized. At block 166, acomplex FFT is performed and at block 168, a predetermined number ofstrong signal target data values from each spectrum are recorded. Atblock 170, the first set of data recorded at block 156 and the secondset of data recorded at block 168 are analyzed to determine varioustarget metrics such as the direction, speed, and range of each target.At block 172, based on the settings the operator has selected,appropriate target data is displayed. For example, the speed of anapproaching target may be displayed in miles per hour. The methodologythen returns to decision block 146 as indicated by the letter A.

FIGS. 6A, 6B, and 6C together depict a further embodiment of a methodfor determining a range of a target. At block 180, the Doppler radarsystem is operating in a moving patrol vehicle, closest target, recedingor approaching mode that is implemented utilizing a relative distancemeasurement process. At block 182, the ground speed tracking flag iscleared. At block 184, the modulation state is set to frequency f₁. Atblock 186, a first buffer of data is acquired and once the end of thefirst buffer is reached, at block 188, the methodology advances to block190 where the modulation state is set to frequency f₂. At block 192, asecond buffer of data is collected. While the second buffer of data isbeing collected, the first buffer of data is processed and analyzed inblocks 194 through 198. At block 194, a Hamming window or other DFTwindow is applied to each half of the first buffer of data in order tominimize end effects. At block 196, a complex FFT is performed utilizingone-half of the buffer as the real magnitude and utilizing the otherhalf buffer as the imaginary magnitude. This generates approaching andreceding spectra of Fourier components. These two spectra are searchedat block 198 in order to find and store a predetermined number of strongsignal target data values and relevant parameters such as frequency,signal strength, and phase value for each target signal.

In one embodiment, the ground speed of the patrol vehicle is required inorder to accurately determine the speed of a target vehicle. At decisionblock 200, if ground speed tracking is enabled, then the methodologyadvances to block 202 wherein the approaching spectrum is searched for anew ground speed target frequency that is equal with or close to thepreviously found ground speed target frequency. At decision block 204,if the new ground speed target frequency is greater than the minimumacquisition value, then a new target to ground speed frequency is savedat block 206. At block 208, the ground speed tracking flag is set. Atblock 210, the methodology waits for the second buffer of data to becompleted.

Returning to decision block 204, if the new ground speed targetfrequency is not greater than the minimum acquisition value, then themethodology advances to block 212 wherein the ground speed timeout delaycount is incremented. At block 214, if the ground speed timeout count isat a maximum, then the process advances to block 216 where the groundspeed tracking flag is cleared before advancing to block 210. Otherwise,the process continues directly from block 214 to block 210.

Returning to decision block 200, if the ground speed tracking is notenabled, then the methodology advances to blocks 218 through 228 inorder to determine the ground speed of the patrol vehicle and activateground speed tracking. In one embodiment, a target that represents thebackground radar signal is found as a representation of the ground speedof the patrol vehicle. At decision block 218, if the vehicle speedsensor is not active, then the approaching spectrum is searched for thestrongest target at block 220. The vehicle speed sensor may be a digitaloutput from the patrol car speedometer, for example. On the other hand,if the vehicle speed sensor is active, then the approaching spectrum issearched for the strongest target within the vehicle speed sensorwindow.

Once the target is acquired in block 220 or block 224, the target istracked to monitor for changes in the ground speed of the patrol vehiclewherein at block 224 if the ground speed target is greater than theminimum acquisition value, the process continues to block 226 where theground speed tracking flag is set and the approaching ground speedtarget frequency is saved at block 228 before advancing to block 210. Ifthe ground speed target was not great enough, then the method advancesdirectly to block 210 and the ground speed acquisition of blocks 218through 228 will have to be performed again.

As previously discussed, at block 210, once the second buffer is full ofdata, the modulation state is set to frequency f₁ at block 230 and thenext first buffer of data is acquired at block 232. While the data isbeing collected, the second buffer of data is processed and analyzed atblocks 234 through 238. The processing and analysis of blocks 234through 238 is similar to the processing and analysis described inblocks 194 through 198, respectively. At block 240, if the ground speedtracking is enabled, then the methodology advances to block 244.Otherwise, the methodology returns to block 188. At block 244, the datacollected at block 198 is compared to the data collected at block 238 todetermine qualifying matches and the range and speed of target vehicles.At block 246, by previous selection by the operator, Opposite Laneclosing speed is displayed by selecting the closest target approachingand subtracting the ground speed frequency from the target frequency. Atblock 248, by previous selection by the operator, Same Lane closingspeed is displayed by selecting the closest target approaching orreceding. If the target is approaching, then the speed is displayed bysubtracting the target relative approaching frequency from the groundspeed frequency. On the other hand, if the target is receding, then thespeed is displayed by adding the target receding frequency to the groundspeed frequency. The methodology then returns to block 188.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is, therefore, intended that the appended claimsencompass any such modifications or embodiments.

1. A vehicular traffic surveillance Doppler radar system, the systemcomprising: a source vehicle; a multiple vehicle environment comprisinga plurality of target vehicles, each of the plurality of target vehicleshaving a range; and a direction-sensing Doppler radar associated withthe source vehicle, the Doppler radar for performing a quadraturedemodulation on reflected modulated FM signals received from at least aportion of the plurality of target vehicles and determining a rangemeasurement for the at least a portion of the plurality of targetvehicles based upon phase angle measurements derived therefrom.
 2. Thesystem as recited in claim 1, wherein the source vehicle comprises a lawenforcement vehicle.
 3. The system as recited in claim 1, wherein thedirection-sensing Doppler radar is operable to determine which of theplurality of target vehicles are approaching and which are receding withrespect to the source vehicle.
 4. The system as recited in claim 1,wherein the source vehicle generates modulated FM signals which aretransmitted to the at least a portion of the plurality of the targetvehicles and reflected therefrom as reflected modulated FM signals. 5.The system as recited in claim 4, wherein each of the ranges of the atleast a portion of the plurality of target vehicles is determined basedupon phase angle measurements associated with the reflected modulated FMsignals.
 6. The system as recited in claim 4, wherein a speed of each ofthe at least a portion of the plurality of target vehicles is determinedbased upon frequency signal differentials associated with the modulatedFM signals and reflected modulated FM signals.
 7. A vehicular trafficsurveillance Doppler radar system, the system comprising: a sourcevehicle; a multiple vehicle environment comprising a plurality of targetvehicles, each of the plurality of target vehicles having a range withrespect to the source vehicle, speed, and radar cross-section; and adirection-sensing Doppler radar associated with the source vehicle, theDoppler radar for performing Fourier transforms on reflected modulatedFM signals received at the source vehicle and determining respectiveranges and speeds of each of the plurality of target vehiclesindependently of the respective radar cross-sections of the plurality ofvehicles.
 8. The system as recited in claim 7, wherein the sourcevehicle comprises a law enforcement vehicle.
 9. The system as recited inclaim 7, wherein the direction-sensing Doppler radar is operable todetermine which of the plurality of target vehicles are approaching andwhich are receding with respect to the source vehicle.
 10. The system asrecited in claim 7, wherein the source vehicle generates modulated FMsignals which are transmitted to the plurality of target vehicles andreflected therefrom as reflected modulated FM signals.
 11. A method foroperating a vehicular traffic surveillance Doppler radar, the methodcomprising: transmitting modulated FM signals from a Doppler radar tofirst and second targets; receiving first reflected modulated FM signalsat the Doppler radar from the first target; receiving second reflectedmodulated FM signals at the Doppler radar from the second target;transforming the received first and second reflected modulated FMsignals to target spectrum; analyzing frequency signal differentialsassociated with the target spectrum to determine respective speeds ofthe first and second targets; and analyzing phase angle signaldifferentials associated with the target spectrum to determinerespective ranges of the first and second targets.
 12. The method asrecited in claim 11, further comprising determining which of the firstand second targets has a faster speed based on the respective speeds ofthe first and second targets.
 13. The method as recited in claim 11,further comprising determining which of the first and second targets iscloser based on the respective ranges of the first and second targets.14. The method as recited in claim 11, further comprising determiningthe respective ranges of the first and second target vehiclesindependently of respective radar cross-sections of the first and secondtarget vehicles.
 15. The method as recited in claim 11, furthercomprising avoiding look-past error by calculating the respective rangesof the first and second target vehicles based upon the phase anglesignal differentials.
 16. A system for operating a vehicular trafficsurveillance Doppler radar, the system comprising: means fortransmitting modulated FM signals from a Doppler radar to first andsecond targets; means for receiving first reflected modulated FM signalsat the Doppler radar from the first target; means for receiving secondreflected modulated FM signals at the Doppler radar from the secondtarget; means for transforming the received first and second reflectedmodulated FM signals to target spectrum; means for analyzing frequencysignal differentials associated with the target spectrum to determinerespective speeds of the first and second targets; and means foranalyzing phase angle signal differentials associated with the targetspectrum to determine respective ranges of the first and second targets.17. The system as recited in claim 16, further comprising means fordetermining which of the first and second targets has a faster speedbased on the respective speeds of the first and second targets.
 18. Thesystem as recited in claim 16, further comprising means for determiningwhich of the first and second targets is closer based on the respectiveranges of the first and second targets.
 19. The system as recited inclaim 16, further comprising means for determining the respective rangesof the first and second target vehicles independently of respectiveradar cross-sections of the first and second target vehicles.
 20. Thesystem as recited in claim 16, further comprising means for avoidinglook-past error by calculating the respective ranges of the first andsecond target vehicles based upon the phase angle signal differentials.